I have so far got these ideas implemented in my third levelpak which is almost finished
To get through a ceiling... (A level where you need to use bombers to get through terrain upwards to the exit)
No looking down (Only 2 floaters, one lemming has to build a bridge to ensure the survival of the other lemmings)
Dead in a few seconds (Lemmings fall several feet after hatching then die, a bridge needs to be built to save the remaining lemmings)
More to think of but there's some ideas
EDIT: Here's two more ideas that I've inserted
Irritable Lemming Syndrome (Release Rate at 99 meaning you have to be carefully clicking on a blocker, during this test I accidentally did the third lemming in the line, but you need to save all but one lemming on the level
Optimism is your answer (A bit tricky, several lemmings die with you needing to build a platform to save them, also to build over a blocker to get to the exit who you made to get the second floater to build)